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\begin{equation*}\textbf{P}_{x_{k}z_{k}} = \sum_{i=1}^N W_{c}^{i}\ [\chi_{k|k-1}^{i} - \hat{\textbf{x}}_{k|k-1}] [\gamma_{k}^{i} - \hat{\textbf{z}}_{k}]^{T} \end{equation*}
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